Orthodontics: Diagnosis, Biomechanics and Retention
Core concepts
Orthodontic diagnosis combines growth assessment, model analysis and cephalometrics. Know the skeletal landmarks: SNA reflects maxillary position, SNB mandibular position, and ANB their relationship - an ANB above roughly 4 degrees suggests a skeletal Class II, a negative ANB a Class III. Angle's classification describes the first molars and is modified by incisor relationship (Class II division 1 versus division 2). Treatment planning rests on anchorage - the resistance of reactive units to unwanted movement - and on biomechanics: the moment-to-force ratio at the bracket determines whether a tooth tips, translates or is torque-controlled. Know the role of functional appliances in growing patients, headgear, temporary anchorage devices, aligner limitations, and the mechanics of overbite correction.
Traps
Questions mix skeletal and dental classification: a Class II molar with a normal ANB is a dental, not skeletal, problem. Others test anchorage loss during space closure, the difference between center of resistance and center of rotation, indiscriminate extraction decisions in borderline cases, and retention - relapse is expected, so retainers are long-term, especially with rotational corrections and expanded arches. Root resorption and gingival recession are recognized risks, not reasons to abandon treatment.
Rule of thumb
Moment-to-force ratio predicts movement: low ratios tip the crown, moderate ratios produce controlled tipping, a ratio near the center-of-resistance distance translates the tooth, and higher ratios torque the root. If the plan demands translation, plan the mechanics and anchorage for it from the start.
Worked example
A 12-year-old presents with SNA 82 degrees, SNB 78 degrees, ANB 4 degrees, and an overjet of 8 mm with proclined upper incisors. This is a skeletal Class II tendency with a dental Class II division 1 pattern. In a growing patient, a functional appliance can influence mandibular position alongside fixed appliances; in a non-growing adult the same numbers would point toward camouflage with extractions or orthognathic surgery, decided by profile, crowding and periodontal limits.